Gases, Liquids and Solids

Bettelheim, Brown, Campbell and Farrell

Chapter 5

• Overview

• Phase Changes

• Gases

• Intermolecular Forces– London Dispersion, Dipole-dipole,

Hydrogen-bonding

• Liquids

• Solids

State characteristics

• Gases– No definite shape or volume; flows– Particles far apart; no interactions

• Liquids– Definite volume, no fixed shape, flows– Particle in contact; moderate interactions

• Solids– Definite shape, volume; does not flow– Particles in contact; strong interactions

Solid Liquid Gas

All particles Particles Particles dotouching touching not touchNo empty Some open Lots of emptyspaces spaces space betweenbetween between themthem themStrong Moderate NoInteractions interactions interactions

Changes of StateMelting: Change from solid to liquid state + heatFreezing: Change from liquid to solid state - heat

Evaporation: Change from liquid to gas state + heatCondensation: Change from gas to liquid state - heat

Sublimation: Change from solid to gas state + heat (Skips liquid state)

Deposition: Change from gas to solid state - heat (Skips liquid state)

Changes of State

• Heat of Fusion: The amount of heat needed to change 1 g of a substance from the solid to the liquid state (at melting point)

• Heat of Vaporization: The amount of heat needed to change 1 g of a substance from the liquid to the gaseous state (at boiling point)

Heat and Phase Changes

• Example: Naphthalene, an organic substance often used in mothballs, has a heat of fusion of 35.7 cal/g. How much heat, in kilocalories, is required to melt 38.4 g of naphthalene?

Heat and Phase Changes

• Example: Naphthalene, an organic substance often used in mothballs, has a heat of fusion of 35.7 cal/g and a molar mass of 128.0 g/mol. How much heat, in kilocalories, is required to melt 0.300 mol of naphthalene?

Phase Changes• The heating curve of ice

Heat of fusion

Heat of vaporization

Kinetic Molecular Theory of Gases

• Gas particles have negligible volume compared to volume gas occupies

• No attraction between particles • Particles move through space in straight lines • Kinetic energy (KE) proportional to temperature (K)• Can collide with container or each other• Total KE before collision = KE after collision• Collisions with walls of container exert pressure

Gases

• Gas pressure:Gas pressure: the force per unit area exerted against a surface– most commonly measured in millimeters of

mercury (mm Hg), atmospheres (atm), and torr

1 atm = 760 mm Hg = 760 torr = 101,325 pascals = 28.96 in. Hg

Gas PressureA mercury

barometer

Gas Laws

• Boyle’s law:Boyle’s law: Volume and pressure are inversely proportional (fixed mass and gas at a constant temperature

PV = constant or P1V1 = P2V2

Gas Laws

Charles’s Law:Charles’s Law: Temperature and volume are directly proportional (fixed mass and pressure); Temperature is in kelvins (K)

VT

V1

T1

V2

T2

= a constant or =

Gas Laws• Gay-Lussac’s Law:Gay-Lussac’s Law: Pressure and

temperature are directly proportional (fixed mass and volume); Temperature in kelvins (K)

PT

P1

T1

P2

T2

= a constant or =

Combined Gas Law• Boyle’s law, Charles’s law and Gay-

Lussac’s law can be combined into one law called the combined gas lawcombined gas law

P1V1

T1

P2V2

T2

PVT

== a constant or

Gas Laws

• Problem:Problem: a gas occupies 2.00 L at 5.00 atm. Calculate its volume when the pressure is 10.0 atm. (Assume no change in temperature.)

Gas Laws

• Problem:Problem: a gas occupies 2.00 L at 5.00 atm. Calculate its volume when the pressure is 10.0 atm. (Assume no change in temperature.)

Gas Laws

• Avogadro’s law:Avogadro’s law: volume of gas is directly proportional to its molar amount at a constant pressure and temperature– Volume number of moles (n)

V/n = constant

– One mole of ANY gas at STP occupies 22.4 L– Standard temperature and pressure (STP)

are 0°C (273 K) and 1 atm pressure

Ideal Gas Law

• Ideal gas law: Ideal gas law: Can be used for a single sample that does not change

PV = nRT

P = pressure of the gas in atmospheres (atm)

V = volume of the gas in liters (L)

n = moles of the gas (mol)

T = temperature in kelvins (K)

R = ideal gas constantideal gas constant (a constant for all gases)

Ideal Gas Law

• The value of R is determined using the fact that one mol of any gas at STP occupies 22.4 L

– Problem:Problem: 1.00 mol of CH4 gas occupies 20.0 L at 1.00 atm. What is the temperature of the gas in kelvin?

PVR =nT

= (1.00 atm)(22.4 L)(1.00 mol)(273 K)

= 0.0821 L•atmmol•K

Ideal Gas Law

– Problem:Problem: 1.00 mol of CH4 gas occupies 20.0 L at 1.00 atm. What is the temperature of the gas in kelvin?

Ideal Gas Law

– Problem:Problem: 1.00 mol of CH4 gas occupies 20.0 L at 1.00 atm. What is the temperature of the gas in kelvin?

Gas Laws• Dalton’s law of partial pressures:Dalton’s law of partial pressures: the total

pressure, PT, of a mixture of gases is the sum of the partial pressures of each individual gas

PT = P1 + P2 + P3 + . . .

• A tank contains N2 at 2 atm and O2 at 1 atm. If we add CO2 until the total pressure is 4.6 atm, what is the partial pressure of CO2?

4.6 atm 2.0 atm 1.0 atm 1.6 atmTotal

pressurePartial pressure

of N2

Partial pressure

of O2

Partial pressureof CO2

+ +=

Intermolecular Forces

• Forces between molecules affect physical properties such as:– Boiling point (bp)– Melting point (mp)– Solubility

Intermolecular Forces

– Occurs because of electrostatic attractions between between positive and negative “poles” of molecules

2 - 10 kcal/mol

1-6 kcal/mol

0.01 - 2 kcal/molIntermolecular forces of attraction

between molecules

London dispersion forces

Dipole-dipole interactions

Hydrogen bonding

Covalent bonds 70 - 200 kcal/molSingle, double, and triple covalent bonds

London Dispersion Forces• London dispersion forces are the attraction between

very temporary induced dipoles

Londondispersion.mov

London Dispersion Forces

– Exist between all atoms and molecules– In general, their strength increases as the

mass and number of electrons in a molecule increases

– Even though these forces are very weak, they contribute significantly to the attractive forces between large molecules because they act over large surface areas

Dipole-Dipole Interactions

• The electrostatic attraction between positive and negative dipoles– consider butane and acetone, compounds of

similar molecular weight

CH3CH2CH2CH3 CH3-C-CH3

O-

+Butane

(bp 0.5°C)Acetone(bp 58°C)

add electron densitymap of acetone

Hydrogen Bonds

• Hydrogen bond: a hydrogen covalently bonded to an atom of high electronegativity (O, N, F) is attracted to another O, N or F

• Affects bp, mp and solubility

H O

H

H

O

H

- +

hydrogenbond

hydrogenbond

- +

(a) (b) (c)

Liquids– When distances decrease so that almost all

molecules touch or almost touch, the gas condenses to a liquid

– The position of molecules in a liquid is random and there is irregular space between them into which other molecules can slide; this causes liquids to be fluid

– Surface tension can result

Evaporation/Condensation

– If a molecule is moving rapidly (has a high KE) and moving upward, it can escape the liquid and enter the gaseous space above it

– Eventually, the number of gaseous molecules will reach an equilibrium with the number of liquid molecules- the partial pressure of the vapor in equilibrium with the liquid: vapor pressurevapor pressure

– Boiling point:Boiling point: the temperature at which the vapor pressure of a liquid equals the atmospheric pressure

Evaporation/Condensation

Solids

– Upon cooling, molecules come so close together and attractive forces between them become so strong that random motion stops and a solid is formed : crystallization crystallization

– Can be crystalline or amorphous

crystallinestructure.mov

Types of Solids

Type Made up of Characteristics Examples

Ionic

Molecular

Polymeric

Network

Amorphous

ions in a crystallattice

high melting points NaCl, K2SO4

molecules ina crystal lattice

low melting points ice, aspirin

giant molecules;can be crystalline, semicrystalline, oramorphous

low melting pointsor cannot be melted;soft or hard

rubber, plastics, proteins

a very large numberof atoms connectedby covalent bonds

very hard; veryhigh melting orcannot be melted

diamond,quartz

randomly arrangedatoms or molecules

mostly soft, can bemade to flow, butno melting point

soot, tar, glass